penn laid out the city, he actually chose a spot of land that had a lot of groundwater. however, by 1730, 30,000 people lived within the first seven blocks of philadelphia, next to the delaware river. well, 30,000 people caused filth in the city and polluted their water sources. the groundwater was not potable. and in one year, 1/6 of the population died of yellow fever. now, they didn't know at the time that yellow fever was carried by mosquitoes. but the health issue was major in that first movement to build a water system. narrator: so they set out to find the cleanest source of water. although the majority of philadelphia's water now comes from the delaware river, early engineers found that development along the waterfront was causing pollution. so their search led them to the nearby schuylkill river. philadelphia developed technologies to pump water from the river into the city.